PWB's are typically made of rigid or flexible layers of insulation materials with alternate printed wiring and/or printed circuit layers that have been bonded together and electrically interconnected with PTH's. PTH's, which are commonly referred to as "vias" or "barrels", form the electrical interconnections between conductive patterns on internal and/or external layers of the printed wiring board. Prior to placing a given PWB through the component assembly operation, it is desirable to test the board, including it's electrical interconnect integrity, to determine whether it is able to withstand the thermal/mechanical stresses experienced during the actual manufacturing assembly process and the product's end use environment.
There are presently a number of accelerated aging methods by which PWB's are tested, with each test generally designed to simulate the stresses that cause the deterioration resulting from natural aging. Thus, such accelerated aging techniques will artificially reproduce and hasten inherent failure modes within the tested PWB. Such aging will have the effect of mechanically straining the layers of the board and the barrel interconnects, due to thermal expansion and contraction. Accordingly, latent defects in the board will be revealed as the cyclic stressing mechanically exercises any weak or defective element to failure. To the extent that the fatigue cycling is done on an accelerated basis, relative to what is normally experienced in manufacturing assembly and in the field, reliability predictions relating to the board can be derived from the test.
Several test methods involve the testing of a specific test "coupon", which is typically a smaller printed wiring board which has been manufactured concurrently on the PWB panel solely for test purposes. Test coupons are manufactured as an integral part of the actual board, and therefore are subject to the same manufacturing conditions and processes as the board with which a given coupon is associated. Therefore, the quality of a given test coupon is a reliable indication of the quality of its associated board, and for all intents and purposes the test coupon is a PWB.
Accordingly, a majority of test methods do not test the board itself, rather they test the test coupon associated with the board. One such test procedure involves thermally cycling the test coupon in a chamber which alternately heats and cools the coupon. This particular approach has been endorsed as the benchmark standard by the Institute for Interconnecting and Packaging Electronic Circuits ("IPC"). The industry standard method has been published in MIL-STANDARD 202F, METHOD 107G, and this standard will hereinafter be referred to by the name "Mil-T", as it is commonly referred to in the interconnect industry.
Although Mil-T cycling is presently sanctioned as the standard by IPC, there are a number of known drawbacks to Mil-T as a test procedure for PWB's. For example, a complete test of a given coupon may take as long as 40 days for the coupon to be processed through the one hundred to one thousand cycles necessary to simulate expected life usage. Further, the operation of a Mil-T chamber can be expensive insofar as the chilled portion of the chamber typically relies upon liquid nitrogen, which is costly to ship, store, and use, and which presents certain environmental concerns.
The Mil-T approach also has the drawback that actual test data relating to a given coupon can be dependent on where the coupon is physically placed in the chamber, which draws into question the "repeatability" of the test procedure. And, test data relating to a given coupon can also be dependent upon which particular chamber the coupon is tested in, which calls into question the "reproducibility" of the test procedure. Finally, the actual operation of a given Mil-T chamber may be technically complex given that a high degree of technical expertise is required in order to insure that all of the coupons in the chamber are cycled to the desired temperature, during the desired time period, and so on.
In order to address the drawbacks of the Mil-T approach, a number of other PWB testing approaches have been developed. For example, one such alternative method heats and cools the coupon by exposing it to a fluidized sand bath. Another method heats and cools the coupon by alternately immersing it in hot and cold liquids, such as oil. Yet a third alternative is generally referred to as the "Power Cycling Technique" ("PCT"). Through the PCT method, DC current is run through the coupon, which causes the coupon to heat up, so that the board and its interconnects are thermally stressed. With respect to all such known methods, however, they likewise have their own drawbacks, relating to factors such as slowness, expense, repeatability, and reproducibility.
Additional drawbacks of known approaches to running DC current through a test coupon relate to the ability of the coupon to uniformly dissipate the heat that is naturally generated during the test. In particular, known coupon designs tend to have rows and columns of interlinked vias, with some such vias positioned on the periphery of the coupon, while others are on the interior of the coupon. Due to the differing heat dissipation properties of the vias positioned on the interior verses the vias positioned on the periphery, some vias will heat to as much as 40.degree. C. greater than other vias which are on the same coupon. Consequently, the hotter, centrally located vias tend to be the ones which are stressed to a significantly greater degree than the cooler ones, and they will naturally fail sooner. Therefore, using coupon designs in which there is generally un-uniform heating results in test data which is naturally limited to those particular vias that were greatly stressed, as opposed to test data relating to all of the vias on the coupon.
Accordingly, what is needed is a PWB test method which addresses the drawbacks of the Mil-T approach, and other known alternate test methods. More specifically, what is needed is a PWB test approach which can achieve identical failure modes/mechanisms and be run in a relatively short time, at a reasonably low cost, with good repeatability and reproducibility characteristics, and which is easy to implement. The test procedure should correlate with the existing Mil-T standard, such that there is a relationship between the cycles of the new test approach and the cycles of the Mil-T approach. Additionally, what is needed is a test coupon that has a generally uniform heat profile, such that the vias on the coupon are roughly the same temperature during coupon testing. Only then are all of the vias going to have approximately the same stresses placed on them, such that all vias are useful for deriving reliable test data.